The present invention relates generally to electronic control systems for air conditioners, heat pumps and refrigeration equipment. More particularly, the invention relates to an apparatus and method using digital circuitry for controlling the heat pump during its normal operating cycle and for performing self-diagnostics to detect refrigerant overcharge and undercharge conditions.
Whether operating as an air-conditioning system or a heat pump system the basic refrigeration cycle operates by metering refrigerant through a closed system in a precisely controlled manner. The refrigerant cools by evaporation in a heat exchanger commonly called an evaporator coil. The refrigerant is metered to the evaporator coil through an orifice sometimes called an expansion valve. Ideally, the refrigeration system should meter just enough refrigerant into the evaporator coil, so that the refrigerant extracts heat throughout the length of the coil as it evaporates. Due to changing dynamics of the system, changes in thermostat settings, changes in load from the sun, wind and so forth, the optimal flow through the expansion valve will need to be varied as the system operates.
Most refrigeration systems employ feedback techniques to sense the system operating conditions and to control the expansion valve so that the desired refrigerant flow through the system is maintained. For example, many systems measure a value called superheat and use this value to control the quantity of refrigerant flow through the expansion valve. Superheat is a measurement of the degree to which the refrigerant in the vapor phase has risen in temperature above the evaporation temperature. In a properly operating system the superheat condition is expected to occur approximately adjacent the exit end of the evaporator coil. Using feedback control techniques, the measured superheat value is compared with a set point value and the error or difference between those two values is used to regulate the refrigerant flow.
Generally speaking the classic feedback control system works well in controlling refrigerant flow in a refrigeration system, provided the system is not undercharged or overcharged with refrigerant. Over time, due to expansion and contraction by temperature change, due to vibration and other physical movement of the system components and due to the eventual deterioration of seals, fittings and components, refrigerant can escape. This results in an undercharged condition. The undercharged condition is characterized by an inadequate quantity of refrigerant. When an undercharge condition occurs, the feedback control system will try to hold the set point temperature, but will be unable to do so because of the inadequate supply of refrigerant. This in turn will cause a rise in system temperatures and results in the general inability to produce sufficient cooling.
The overcharge condition is just the opposite of the undercharge condition. The overcharge condition occurs when a maintenance person or homeowner puts too much refrigerant into the system (i.e. more than that recommended by manufacturer specifications). In an overcharge condition the system will try to maintain the set point temperature. However, due to the excessive charge, the resultant temperature will be too low, leading to inefficient operation, system freeze-up and undue wear on system components resulting from floodback of compressor thereby affecting the reliability of the system.
In a conventional refrigeration system the onset of an undercharge condition will probably go undetected until the system is no longer able to provide adequate cooling. In such cases the home or building is left without air-conditioning (quite often during the hottest days of the year) until a service call can be made.
Detecting an overcharge condition in the conventional system is even more difficult, since the homeowner or building occupant may not notice any problem (other than possibly noticing higher electric bills) until the system shuts down altogether due to component (compressor) failure.
Thus it would be very desirable to provide a foolproof and convenient way of checking for both undercharge and overcharge conditions and of providing an early warning to homeowners and building maintenance personnel. The present invention provides such a system. It is integrated into the refrigeration flow control system and is able to detect both overcharge and undercharge conditions. As more fully explained below in connection with the detailed description of the presently preferred embodiment, the present invention detects the undercharge condition by monitoring the setting of the expansion valve and monitoring the compressor discharge temperature. The system uses compressor discharge temperature as a measure of superheat.
The system tests for the overcharge and undercharge condition by periodically placing the expansion valve in a predetermined known setting and by then measuring the resultant compressor discharge temperature. The measured temperature is compared with a reference value stored in a look-up table. If the measured temperature is out of tolerance, that is, below a predetermined minimum value, then an overcharge condition is declared. Similarly, if the discharge temperature is above the reference value, then an undercharge condition is declared.
In accordance with one aspect of the invention a method of diagnosing an undesired refrigerant charge condition is provided. The method involves storing at least one temperature reference value and at least one valve setpoint value. In the preferred embodiment tables of temperature reference values and valve setpoint values are utilized. According to the method, a quantity indicative of environment temperature is measured to determine a first measured temperature value. The expansion valve of the refrigeration system is then caused to attain a predetermined setting, based on the valve setpoint value. While the valve is at the predetermined setting, a quantity indicative of refrigeration system temperature is measured to determine a second measured temperature value. In the preferred embodiment the first measured temperature value is based on outdoor air temperature and the second measured temperature value is based on compressor discharge temperature. The method proceeds by using the first measured temperature value to access the stored temperature reference value and comparing the accessed temperature reference value with the second measured temperature value. An undesired refrigerant charge condition is declared (e.g. overcharge or undercharge) when the accessed temperature reference value and the second measured temperature value are not within a predefined numerical proximity to one another. If desired, the undesired refrigerant charge condition is declared as an overcharge condition when the second measured value is less than the temperature reference value. Similarly, an undercharge condition is declared when the second measured value is greater than the temperature reference value.
In accordance with one aspect of the invention a method of diagnosing a loss of refrigerant charge condition is provided. The method is effected by storing a temperature setpoint value and a valve limit value. A quantity indicative of the refrigeration system temperature is measured to determine a first measured value and the setting of the expansion valve is monitored to determine a second measured value. A loss of refrigerant charge condition is declared when (a) the second measured value is in a predefined proximity to the valve limit value and (b) the first measured value is outside a predefined proximity to the temperature setpoint value.
According to another aspect of the invention a method of diagnosing a refrigerant overcharge condition is provided. The method is effected by storing a temperature setpoint value and a valve setpoint value and periodically causing the expansion valve to attain a setting determined by the valve setpoint value. In the presently preferred embodiment a fully open setting is preferred, although other settings can also be employed. While the expansion valve is at the setting determined by the valve setpoint value, a quantity indicative of refrigeration system temperature is measured and a measured value is thus determined. An overcharge condition is declared to exist when the measured value is outside a predefined proximity to the temperature setpoint value.
Yet another aspect of the invention is a method of diagnosing an incorrect charge condition. The method is effected by storing a temperature setpoint value and a valve target position value. The setting of the expansion valve is monitored to determine a valve setting value. This valve setting value is compared with the valve target position value. When the valve setting value and the valve target position value are within a predefined proximity to one another a quantity indicative of refrigeration system temperature is measured to determine a refrigeration value. An incorrect charge condition is declared to exist when the refrigeration value is outside a predefined proximity to the temperature setpoint value.
For a more complete understanding of the invention, its objects and advantages, reference may be had to the following specification and to the accompanying drawings.